The present invention relates to slow blowing fuses of the type which commonly comprise a cylindrical insulating housing having metal terminal-forming end caps between which extend within the housing a fuse body including fuse wire spirally wound upon a support core which acts as a heat sinking body for extending the time it takes the fuse to blow when a current of a given value flows through the fuse wire. Slow blowing fuses are utilized in environments where the electrical circuit in which the fuse is located is not to be interrupted by blowing of the fuse unless an undesired level of current flows for a given minimum length of time.
The cores upon which the fuse wire has been heretofore wound have taken a number of different forms. For example, as disclosed in U.S. Pat. No. 2,672,540 granted Mar. 16, 1954 to C. G. Dewey, the core comprises a rigid body of ceramic material over which the fuse wire is wound. The main disadvantage of such a rigid ceramic core material is that a rigid body cannot be wound into rolls, and so must be individually supported and handled during the fuse assembly procedure, increasing the cost of manufacturing such fuses as compared, for example, to a fuse body construction where the core is made of a windable, flexible material which can be wound into rolls, as, for example, disclosed in U.S. Pat. No. 2,879,364, granted Mar. 24, 1979 to G. J. Mucher. Thus, where a flexible core material can be wound into rolls, fuse bodies can be readily mass produced by unwinding the core material and then spirally winding the fuse wire therearound, and either immediately severing individual fuse bodies from the end of the fuse wire, or rewinding the filament wire wound core of material into rolls and then subsequently unwinding the rolls and severing the fuse bodies therefrom during the process of assembling a complete slow blowing fuse (where each fuse body is enclosed in and soldered to end caps of a housing then sealed from the surrounding atmosphere).
In the manufacture of cores of a material comprising filament of a material like fiber glass, the cores are formed from twisted strands of the material involved. To maintain the integrity of such twisted strands of material, the individual strands are held together by a suitable binding material referred to as "sizing", which is generally a synthetic resin material. Unfortunately, the temperature conditions occurring during the blowing of a fuse having a body made of fuse wire wound around such a sizing-containing core results in the carbonization of the binding material, which leaves a coating of conductive carbon along the core. Many fuse applications require a very large insulating resistance between the terminals of the fuse when the fuse is blown, and so it has been found that the carbonization described frequently reduces the insulating resistance between the terminals of the fuse below the high desired insulating resistance now commonly required for such fuses.
It is, accordingly, an object of the present invention to provide a method of making the same where the core material upon which the fuse wire is wound is one which can be readily wound and unwound from a spool and which does not result in carbonization under fuse blowing conditions, so that a high insulation resistance is present after a fuse made therefrom is blown.